1. Field of the Invention
The present invention relates to an information recording system.
2. Description of the Prior Art
Along with the increase in the capacity and the operation speed of the electronic computer in recent years, high density and large capacity for the memory devices are being demanded to an increasingly high degree. Examples of memory devices include an optical disk file device which records and reproduces information as a bit sequence that can be read optically, a magnetic recording device which carries out recording of information on a recording medium with magnetic layers by inverting and an erasable magneto-optical disk device which records and erases information by means of irradiation of laser light and application of a magnetic field, and reproduces information by irradiation of laser light. Among various memory device, one which is attracting the most attention is the information recording and reproducing device that makes use of an optical disk. The optical disk device records information by irradiating an optical disk with a laser beam that is squeezed to a spot with diameter of about 1 m to form a bit that has a width of 0.6 to 1 m and a length of 1 to 2 m. The optical disk device reproduce recorded information by detecting the changes in the reflected or transmitted light from the bits recorded on the optical disk.
Examples of optical disk recording media include a recording medium of direct read after write (DRAW) type for which it is impossible to erase and re-record information, and a recording medium of erasable type for which it is possible to re-record (erase or record) information for a plurality of times. The information recording and reproducing DRAW type device has an advantage of impossibility of rewriting information. However, from the viewpoint of economy and effectiveness the erasable type is more superior.
A recording medium of erasable type has an amorphous alloy film that is composed of rare earth elements such as Gd, Td, Dy, and Ho and transition metals such as Fe and Co, as the recording film. From the magnetically ordered condition maintained at the room temperature, with the easy axis of magnetization in the direction perpendicular to the magneto-optical film, a magnetically disordered state can be obtained by irradiating the recording film (perpendicularly magnetized film) with a laser beam or the like.
The magnetically ordered and disordered states here mean the conditions in which the coercive force of the easy axis of magnetization is securely held and decreased, respectively. If an external magnetic field with a predetermined direction is applied to the recording film when it is in a magnetically disordered state, the easy axis of magnetization of the recording film rotates toward the direction of magnetic field applied. In this way, it becomes possible to carry out recording or erasing.
Now, the method of recording information on the recording film that has all of its magnetic domains arranged in the same direction (for example, in the downward direction) in a state where is no information is recorded, will be described in detail.
A laser beam which modulates information, is focused to a size with diameter of about 1 m, and is made to irradiate the recording film to heat it locally with the energy of the laser beam to reduce the coercive force of that portion of the film. In this case, by applying an external magnetic field (usually, less than 1 kOe) with the direction which is opposite to the initial direction (downward direction), of magnetization the local direction of magnetization alone will be reversed. The portion of the recording film other than the portion that is irradiated by the laser beam will not undergo a change in the initial direction of magnetization (inversion) even under the application of an external magnetic field. Therefore, there will be formed a pattern that has different direction (upward direction) of magnetization within a uniform field of magnetization. That is, it corresponds to a recording of information.
Next, a method of erasing previously recorded information by means of a principle which is nearly the same as in the above will be described. Namely, one needs only to select the direction of magnetic field to be opposite to the direction of the magnetic field that was applied at the time of recording information. For instance, by irradiating the recorded portion of the recording film with a laser beam and by applying an external magnetic field with direction which is opposite to that at the time of recording, the direction of magnetization returns to the state which is the same as prior to the recording, erasing information previously recorded.
Further, for reproducing information, differing from the case of recording and erasing, use will be made of the rotation of the plane of polarization of the laser beam. On the recording film (perpendicularly magnetized film) of the recording medium, there are disposed magnetic domains perpendicularly. When the film is irradiated by a laser beam, the direction of polarization of the reflected (or transmitted) light rotates according to the direction of magnetic domain. In other words, if a linearly polarized laser beam irradiates the surface of the film, the plane of polarization of the reflected light rotates slightly depending upon the direction of magnetization of the film. The rotation of the plane of polarization is detected by means of a light detector and the signal due to the direction of magnetization of the film is converted to the intensity (1 or 0) of the reflected light, reproducing the information.
In the above, recording, reproducing, and erasing of information are performed by making use of the sence of magnetization of each magnetic domain.
As a memory with the recording film having the above features, the erasable medium is called a magneto-optical disc. Although this magneto-optical disc is erasable, it is inappropreate to carry out writing operations directly on the area which has already undergone the previous writing operations, since the domain inversed by the previous operations is left as it was. Because of this, erasing operations where writing is to made is necessary in advance before new information can be written on the area. Of cource where only one head is available, it takes double the time to complete re-recording.
One method for rewriting of information, for example, is a magneto-optical device for recording, reproducing, and erasing disclosed in Japanese Patent No. 59-217250. This device has an optical system for recording that records of a desired information, and another optical system that erases unnecessary information or reproduces information that was written by means of the recording optical system. With such an arrangement, it becomes possible to partially erase unnecessary information alone, and at the same time, to record new information.
However, in attempting to apply such a concept to a practical device, there still remains certain unsolved technical problems. For example, 1. Provision of two heads requires large sized hardware and sophisticated control systems. 2. It can perform only signal reproduction that is entirely identical to the case of an ordinary single head optical system. 3. Sufficient response is not available for information that comes transported successively (continuously flowing information). 4. The waiting time for rotation, in accessing the optical system to the position where rewriting is desired, is too long. 5. Finally, the size of the recording bits for such a device is extremely small, being on the order of one micrometer. Accordingly, even dust, scratches, pinholes, and other small size objects could become the cause of the faulty signals in recording or reproducing, which results in larger error rates compared with the case of magnetic disks. Thus, the magneto-optical disk device is not put to practical use unless these technical problems are resolved.